A seat belt system for restraining an occupant of a vehicle ordinarily includes seat belt webbing, a buckle and a retractor. A locking tongue is connected to the webbing and is releasably lockable in the buckle when the webbing is extended around the vehicle occupant. The retractor includes a spool upon which the webbing is wound. The spool in the retractor rotates in an unwinding direction when the vehicle occupant withdraws the webbing from the retractor and moves the webbing toward the extended position in which the tongue is locked in the buckle. When the tongue is subsequently unlocked and released from the buckle, a spring in the retractor rotates the spool in a winding direction to draw the webbing back into the retractor, and thus to move the webbing back from the extended position to a retracted position.
When the vehicle experiences a collision, the vehicle occupant wearing the seat belt webbing moves forcefully against the webbing. The force which is then applied to the webbing by the vehicle occupant urges the webbing to move outward from the retractor. Therefore, a retractor in a seat belt system ordinarily includes a lock-up mechanism which prevents the spool from rotating in the unwinding direction when the vehicle experiences a collision. Several different types of lock-up mechanisms are known. One type of lock-up mechanism is associated with the buckle. That type of lock-up mechanism is actuated by the tongue on the webbing upon locking of the tongue in the buckle. For example, one such lock-up mechanism is disclosed in U.S. Pat. No. 4,720,148.